Hall Effect sensors have been used in measurement devices such as thickness gages (e.g. Olympus NDT Magna Mike 8500) to accurately measure thickness of nonferrous materials. One of the most often seen applications is thickness measurement on plastic bottles. A Hall Effect sensor typically comprises a probe that has magnet(s) generating a primary magnetic field. Measurements are performed by holding the device's magnetic probe to one surface of the test material and placing a small steel target ball on the opposite surface. The target ball, in responding to the primary magnetic field, generates a secondary magnetic field, which varies according to the distance between the probe and the steel target ball. A Hall-effect sensor, which measures the strength of the secondary magnetic field, built into the probe measures the distance between the probe tip and target ball. Typically measurements are instantly displayed as easy-to-read digital readings on the device display panel.
One unique challenge encountered and overcome by the present disclosure involves a hall sensor that is not part of an integrated circuit on board the instrument. As required by many Hall Effect instruments or applications, a major portion of the circuitry is assembled on the main body of the instrument, which is coupled to the Hall Effect sensor or probe via wires or cables with a length that meets the operator's needs, e.g. 1 meter. This physical distance between the Hall Effect sensor and the instrument presents an unknown wiring and connector resistance. As the Hall Effect sensor is located in the probe and sensitive to temperature changes, it presents a unique challenge for the instrument to compensate the temperature of the probe assembly including both magnetic parts and the Hall effect sensor.
It also presents more unique challenges when the operation of a Hall effect sensor based instrument involves interchange of Hall sensor probes and gage and maintaining an accurate and temperature compensated system.
However, it's been widely observed that the accuracy of a measurement from a Hall effect thickness gage drifts with temperature quite noticeably. It is also known that the resistance of the Hall Effect sensor varies with temperature. Because the measurement is directly related to the resistance of the Hall Effect sensor, a change in temperature would result in a change in the Hall Effect resistance and a change in the result of the magnetic measurement. This is also called measurement drift due to temperature.
An existing effort made in an attempt to reduce this effect was to re-calibrate the instrument whenever the instrument is in a condition called “Ball-Off condition”, i.e. whenever there are no targets. By re-calibrating, adjustment is made so that the sensor is calibrated to the current testing conditions, including temperature. However, since this Ball-Off condition does not always occur, or occur frequently enough, the measurement could drift with temperature change without the knowledge of measurement taker or operator.
Another existing effort has been seen in patent U.S. Pat. No. 5,055,768 in which a temperature sensitive current source is deployed to solve the problem of Hall effect sensor sensitivity to temperature. This current source is intended to be part of the Hall effect sensor. However, the circuit as disclosed is limited to compensating temperature effects inside the Hall sensor residing on the same chip.
Yet another existing effort seen in U.S. Pat. No. 6,281,679 involves a system that uses a magnet and a Hall Effect sensor to measure distance. However, the magnets and the Hall sensor move in relation to each other. It teaches a method by which two Hall sensors are matched so that temperature is not a factor. It also addresses methods of regulating the temperature of the magnet and Hall sensors by auxiliary temperature control, including using circulated air. Yet, it failed to mention the challenge brought by and hence the solution to the issue of temperature variation between the locals of Hall probe and the processing circuit, which is located in the instrument.
U.S. Pat. No. 8,274,287 uses a magnet and a Hall sensor to detect disturbances in the field. It also employs a temperature sensor to control the temperature compensation of its measurement on quantity of metallic debris. However, the patent did not make use of the unique property and the subsequent advantages presented by Hall sensors' sensitivity to temperature. It did not make any effort in measuring changes of Hall sensors circuitry reading attributed to temperature change. In addition, it explicitly regards the temperature response as linear, which is not an accurate representation of this line of Hall sensor devices.